A useful information item can generally be transmitted between a transmitting terminal and a receiving terminal in a plurality of transmission formats. For example, a useful information item can be transmitted by using a number of possible modulations (BPSK, DBPSK, OPSK, 16QAM, etc.) and/or a number of possible types of error correcting codes and/or a number of possible redundancy levels, etc. Furthermore, it is also possible to allow the transmission of useful information items of different sizes.
In order to facilitate the reception of a useful information item by the receiving terminal, it is known practice to transmit, between the transmitting terminal and the receiving terminal, control information items relating to the transmission format used by the transmitting terminal (size of the useful information item, modulation, error correcting code, redundancy/punching rate, etc.).
It is understood that, if the control information items are not correctly interpreted by the receiving terminal, the transmission format used by said receiving terminal to interpret the useful information item will be different from the transmission format actually used by the transmitting terminal, such that the useful information item will not be correctly interpreted by said receiving terminal. To remedy this problem, the control information items are generally made reliable, for example by using a modulation with predefined low spectral efficiency and/or by using an error correcting code with a predefined high redundancy level, etc.
Furthermore, in a multiple access network, in which multiple and independent transmitting terminals have the initiative for the moment of transmission, means have to be implemented to make it possible to accurately detect, in a data stream, the start of the useful information item. As is known, the preamble contains a synchronization word, called “frame synchronization” word, identifiable by all the receiving terminals. For example, each receiving terminal which receives the data stream performs a sliding correlation of the frame synchronization word over the data stream. When the maximum result of the correlation exceeds a predefined threshold, it is considered that the frame synchronization word is present in the data stream, at the position that made it possible to obtain said maximum result. The detection of the position of the frame synchronization word enables the receiving terminal to know the start of the useful information item in the data stream.
In the transmission of a useful information item between a transmitting terminal and a receiving terminal, the useful information item can thus be preceded by a preamble containing both a frame synchronization word and control information items. The size of the preamble can be considerable compared to the size of the useful information item, especially for short useful information items and/or when the control information items have a high redundancy level for reliability purposes. Now, the effectiveness of the use of a frequency band decreases as the ratio between the size of the preamble and the size of the useful information item increases. This is all the more detrimental if the frequency band used for the transmissions is narrow, for example of the order from a few hertz to a few hundreds of hertz.